Jet-black means as deeply black as it is possible to be. Jet-black hair, jet-black eyes, a jet-black coat.
The jet in jet-black is a real material, a hard black stone formed from ancient wood, which was carved into Victorian mourning jewellery. Whitby in Yorkshire was the centre of that trade.
1adjectivejet-blackAs deeply black as it is possible to be.
“The cat had jet-black fur and bright green eyes.”
jet-black is a colour.
Also accepted in games: jet black
From jet, a hard black stone formed from fossilised wood, plus black.
From jet + black.
Etymology from Wiktionary, CC BY-SA
Practised in A Song of Ice and Fire